An approach to dissecting the processes of endocytosis, glycoprotein biosynthesis and sorting is through isolation and analysis of mutants. Based on observations of aberrant compartmentalization of the M6P/IGFII receptor in a temperature-sensitive End1 mutant, perturbation of the intracellular pathways followed by this receptor was attempted. Summarizing these results: 1. In the presence of weak base human diploid fibroblasts accumulate up to 40% of receptor-enzyme complexes in dense lysosomes; this accumulation represents a steady state and is rapidly reversed upon re-acidification of organelles. 2. Inhibition of protein synthesis effects rerouting of lysosomal enzymes present in the secretory pathway from a predominantly intracellular to a predominantly endocytic route of delivery to lysosomes. 3. Inhibition of protein synthesis also blocks accumulation of receptor-ligand complexes in lysosomes and effects a futile cycle whereby these complexes shuttle between the cell surface and an intracellular compartment(s). It is hypothesized that a rapidly turning-over protein is required for delivery of receptor-enzyme complexes to the late endosome via both the endocytic and intracellular pathways. Electron microscopic examination of endocytosed tracers in a CHO cell mutant isolated for its accumulation of ligand in non-acidic vacuoles indicates that the mutant may be impaired in vesicular fusion.